An on-line ultrasonic velocity gauge is the subject of U.S. Pat. No. 4,291,577 Baum et al., the disclosure of which is herein incorporated by reference. The invention of Baum provides an ultrasonic transducer in each of two wheels that ride on a moving web and that are axially aligned across the width of the web. The circumferential positions of the transducers relative to their respective wheels are common so that the transducers contact the web at substantially the same time. The wheels have mercury slip rings for electrical communication to an external control unit which apparently serves as both a signal processor and power conditioning means. As explained in the description of the invention, the slip rings generate electronic noise which makes it difficult to obtain signal to noise ratios that are sufficiently large to enable accurate measurement. In addition, precise measurement of ultrasonic propagation time is difficult to achieve with only one receiving transducer because its electro-acoustical properties may vary with changes in environmental conditions such as temperature, and because electro-acoustical delays in the transducer itself make it difficult to accurately determine propagation time. One can compensate for the latter problem by providing two receiving transducers along each line of measurement that are positioned at different distances from the source of ultrasound. This is exemplified in U.S. Pat. No. 4,574,634 Pappano, the disclosure of which is herein incorporated by reference.
If one desires to use existing systems to measure the speed of ultrasound at several zones across the width of the web, then at least two major problems are presented. First, if the existing system accomplishes the desired result through the use of a conventional web-traversing mechanism, the physical contact between the web and the rotating wheels may cause web damage and machine downtime. Second, if several wheels are used with a single transducer in each wheel, the formentioned problem of low signal to noise ratios is compounded.
An important object of this invention is to provide apparatus and methods for eliminating major sources of electronic noise in systems which measure the speed of ultrasound in a moving web of paper.